I bought some from an online retailer...I own a Sundance Cameo, but saw no reason to purchase the Sundance brand filters since the type of filtering would not be intrinsic to a brand. I paid about $10 per filter, which are supposedly good for about 3 fills.
My first fill had CH levels near 450...when I filled it the second time, utilizing a pre-filter, my CH levels have been closer to 275...maybe a bit higher than ideal but notably lower. I see less buildup near the waterline and on some of the jets as a result, so I found the purchase of the pre-filters to be a good one. Just my 2 pennies!